The present invention relates to apparatuses for manufacturing semiconductors and more particularly to photolithography apparatuses.
In the field of semiconductor manufacturing, photoetching steps are used at numerous manufacturing steps.
A photoetching step conventionally comprises the following steps:
depositing on a substrate the thin layer that is desired to etch in order to leave some portions in place,
depositing on this thin layer a photosensitive masking layer, usually called resist,
insolating the resist according to the determined pattern,
developing the resist for leaving in place the insolated portions (positive resist) or the non-insolated portions (negative resist), and
etching the thin layer at the places non-covered by the resist.
Also, in the field of semiconductors, numerous processes have been developed for etching thin layers. Among those processes, can be cited: wet etching processes (in an etching bath) and dry etching processes (in pre-sence of a plasma) as well as anisotropic and isotropic etching processes.
Whatever the etching process may be, one of the key steps of this process is the insolation of the resist layer, that can be carried out by means of various known apparatuses, for example a photorepeater. Indeed, if the mask image that is desired to be formed on the resist layer is not properly focussed (too short focussing) or if the insolation dose is insufficient, the photoresist is liable to be underexposed which will cause it to remain partially in place after development in areas from which it should have been removed from. Conversely, if insolation is excessive, or if the depth of focussing is too high, the resist will be overexposed and, during the resist development phase, the sizes will be higher or lower (positive or negative resist) than the desired sizes. In the modern technologies where it is desired to define lines having a width of about one micrometer, this focussing step is particularly critical and variations in temperature or hygrometry of the room where the apparatus is located may cause modifications of the setting of said apparatus. Therefore, the apparatus has to be periodically reset, usually at least once a day, in an assembly line.
Conventionally, in order to carry out this setting, a same test pattern is imaged onto a test wafer, in a repetitive and successive way in various places of the wafer, corresponding for example to the various positions of the chips it is desired to form on the wafer during the normal manufacturing process. For each place where the pattern is imaged, focussing of the exposure machine is liable to vary by a very low value with respect to the average value. Then, after development of the resist, each of the patterns is observed by means of a high-power (500-1000) microscope for selecting the one that corresponds to the optimal resolution, that is, the first pattern wherein the resist is completely removed at the desired places without overetching the resist. This operation is delicate for the operator because the focussing depth of the microscope is lower than the depth of the resist (for example of about one micrometer for the whole layer depth) and, on the other hand, it is inaccurate since it depends upon the interpretation that can be made by the operator. Of course, it is possible to improve the accuracy by using a scanning electron microscope but this involves an increase in measurement time and cost.